Digital disk drives are widely used in computer systems to permit rapid storage and retrieval of information. Such devices are typically comprised of one or more co-rotatable disks, one or more movable write/read heads and an electronics assembly. The rotatable disks have invisible concentric tracks, to or from which information is written and read using the movable magnetic write/read heads. The write/read heads are commonly mounted on an arm wherein a motor moves the arm either linearly or arcuately. Often such a motor is a voice coil motor (a "VCM") or a moving magnet motor (a "MMM"). The stator of the VCM is mounted to a base plate or casting on which is mounted a spindle supporting the disks. The base casting is in turn mounted to a frame via a compliant suspension. When current is fed to the motor, the VCM develops force or torque which is substantially proportional to the current, wherein the constant of proportionality is known as the actuator force or torque constant. The arm acceleration is therefore substantially proportional to the magnitude of the current. The constant of proportionality is known as the actuator acceleration factor (the "aaf"), and is equal to the actuator force or torque constant divided by the moving mass or inertia respectively. The force or torque developed by the motor is also experienced in an equal and opposite sense by the VCM stator and the base casting.
The two primary objects of a head positioning system are (1) to maintain the head at or very near the track centerline while writing or reading and (2) to move the head rapidly from one track to another so as to minimize the time taken to locate the head at or near the centerline of a target track. The latter function is known as "seeking". Typically, a head must be located with an accuracy of the order of .+-.10% of the distance between the centerlines of adjacent tracks (the "track pitch"). In the current art, the track pitch may be of the order of 500 microinches, which means the head must be located within approximately .+-.50 microinches of the track centerline for successful read or write operations to take place. Therefore, at the end of a seek, the head must "settle" to within .+-.50 microinches of the target track centerline before write or read operations are permitted. As track densities become higher, the settling requirement becomes increasingly onerous.
All disk drives today have a relatively high density of concentric information tracks (almost invariably greater than 1000 tracks per inch). Accordingly, an accurate servo control system is required to position the head on the track centerline and to move the head from one track to another. Such a control system typically depends on position information recorded on one or more of the disk surfaces. There are many types of position information systems used but three basic categories exist; (1) the dedicated servo surface, (2) servo sectors, and (3) a combination of (1) and (2). A description of the dedicated servo concept may be found in MUELLER, ET. AL., IBM TECHNICAL DISCLOSURE BULLETIN, Vol 21, No. 2, 804, 805 (February 1978) hereinafter "MUELLER"!. A description of sector servos may be found in Lewis, et. al., U.S. Pat. No. 4,424,543, issued Jan. 3, 1984. A description of a system employing a combination of dedicated and sector servos may be found in Case, et. al., U.S. Pat. No. 4,072,990, issued Feb. 7, 1978. It is to be noted that the present invention applies equally to disk drives utilizing dedicated servo, sector servo and combinations of the two.
The highest performance positioning systems today utilize digital control systems. The general theory of such systems is described in G. FRANKLIN, J. POWELL, & M. WORKMAN, DIGITAL CONTROL OF DYNAMIC SYSTEMS, (2d ed. 1990) hereinafter FRANKLIN!. Several patents disclose digital control systems with particular application to disk drive head positioners. See, e.g., Workman U.S. Pat. No. 4,679,103, issued Jul. 7, 1987, Edel, et. al. U.S. Pat. No. 4,835,633, issued May 30, 1989, and Takita U.S. Pat. No. 4,954,907, issued Sep. 4, 1990. Each of these systems employs a state estimator as described generally in Chapter 6 of FRANKLIN, supra, at 250.
One problem with current head positioning systems is the impairment of settling time due to the reaction force or torque experienced by the VCM stator and the base casting. This force or torque causes the casting, and therefore the target track on the disk, to move with respect to the arm supporting the head. This motion has dynamics related to the characteristics of the suspension between frame and casting. These motions are sometimes referred to as internally induced motions or self-excited motions to distinguish them from motions caused by external shock or vibration. The suspension typically behaves as a lightly damped second order resonance. Consequently, seek lengths of particular time durations will, even after the VCM force or torque has subsided, elicit sympathetic post-seek oscillatory motions between head and track. Very often, the servo loop bandwidth is insufficient to track these motions, resulting in an extended seek time.
The phenomenon of such post-seek motions is discussed in FRANKLIN, supra, at 741, 742. The discussion proposes the implementation in the state estimator of a model of the structural resonance of the disk drive which model, at disk start-up, is adapted to track the actual resonance of the drive. Previous proposals directed to this approach have utilized accelerometers and include Knowles U.S. Pat. No. 4,775,903, issued Oct. 4, 1988, and an article entitled "Design Strategies for High-Performance Incremental Servos" by Martyn A. Lewis, Proceedings of the Sixth Annual Symposium on Incremental Motion Control Systems Society, at 143, 144, May 1977, Department of Electrical Engineering, U. of Ill., Urbana Champagne, Ill. However, these methods require costly accelerometers to measure the post-seek displacement of the head. Furthermore, in the method disclosed by Knowles, the acceleration profile and track position trajectory signals were sampled for each duration of seek length. Each of these sampled values had to be stored in memory and accessed through a "look-up" procedure at the beginning of each new seek operation.
The use of a mathematical model of the physical plant to predict the post seek displacements has inherent limitations in practice. First, accurate models involve intensive computations which are inappropriate for the low-cost microcontroller ordinarily found in disk drives. Secondly, such a model is necessarily limited to the particular plant structure for which it is developed and must be modified to account for changes in the physical construction of the disk drive. Finally, a model is at best an abstraction from reality, and the effectiveness of even a perfect model of the physical plant will be undermined by variations in the characteristics of the other components of an actual disk drive.
The traditional approach to modelling the reaction of a physical system such as a disk suspension has been to accumulate through sampling, a time series of control inputs and the corresponding outputs or dynamic response, and from this data attempt to construct or derive a transfer function which accurately describes the relationship between the input and the output states of the system. In the case of a disk drive, such an approach would entail collecting the position signals generated by the control circuitry, and matching those to the observed resonance of the suspension system, as measured by sampling the position error signals. However, during a seek operation, the acceleration of the head relative to the track position is dominated by the dynamic response characteristics of the positioning motor coil. The relationship between the control input and the response of the suspension can only be observed after the control input has subsided below a certain threshold. Furthermore, a mathematical model which accurately describes the observed relationship involves transcendental elements and is computationally intensive. Thus, it is desirable to employ a method which requires only that the position error samples be collected and analyzed, and which minimizes the required computations.
An entirely different technique is described in Dunstan, et. al. U.S. Pat. No. 4,947,093, issued Aug. 7, 1990. This patent disclosed a method which uses the frequency modulation of the clocking information on a dedicated servo surface as an indication of the vibrations due to internal and external sources. After signal processing, the frequency modulation is fed forward to the VCM to compensate for the resulting track displacements. This method results in a signal to noise ratio which is less than may be desired. Furthermore, the Dunstan approach requires a clock of relatively high frequency which may not be available in a disk drive utilizing a pure sector servo method of positioning the head rather than a dedicated servo surface. In contrast, the present invention is applicable to both dedicated servo and sector servo systems.
Accordingly, the primary object of the present invention is to provide an effective, inexpensive means of adapting the positioning mechanism of an individual disk drive to minimize the displacement of the head due to internal accelerations. A further object is to provide a general method of compensating for post seek displacements which is independent of the physical structure of the disk drive and which minimizes the computational requirements placed on the control system. This approach will allow the design of the suspension to be optimized with respect to externally induced vibrations rather than being compromised to respond to both internally and externally induced vibrations.